THE BIG SWAP

Replace the metal gate with a liquid

Gate metal → solution

An ion-sensitive field-effect transistor, or ISFET, removes the metal gate. The insulator's surface is exposed to a water-based solution, and a reference electrode dipped into that solution supplies the gate voltage. Piet Bergveld introduced the idea in 1970.

It is the same transistor with a different gate: a liquid.

ISFET CROSS-SECTION

The gate stack, rebuilt

Source, drain, and body are unchanged. Going up from the silicon, the gate stack is now an insulator, then an electrolyte, then a reference electrode. The gate voltage goes to the reference electrode, and the solution passes it down to the insulator surface.

The channel cannot tell whether its gate is metal or liquid, only what potential it sees.

THREE NEW PARTS

What the gate stack contains

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Electrolyte

Water with dissolved ions. It conducts, so it can carry the gate voltage

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Reference electrode

Holds the solution at a stable, known potential

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Sensing insulator

The oxide surface that touches the liquid and reacts with it

The reaction at that last surface is where the sensing happens.

WHAT STAYS THE SAME

The MOSFET rules still apply

Channel Forms and conducts exactly as in a MOSFET

I–V behavior Cutoff, triode, and saturation, with the same equations

Threshold logic A shifted threshold is still a shifted curve

Readout Transfer curves and transconductance work unchanged

Everything you know about MOSFETs is now a tool for reading a solution.

THE KEY IDEA

One term depends on the solution

Vth = constants − ψ₀

An ISFET's threshold has many fixed terms: the reference electrode, the silicon work function, oxide charge, and more. Only ψ₀, the potential at the insulator surface in contact with the solution, responds to the chemistry. A threshold shift is therefore a direct reading of ψ₀.

The transistor turns a surface potential into a current.

UNIT 8 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered swap the metal gate for a solution and a reference electrode.